The P4 was on a different front line, that's why it wasn't a target for me from the beginning. As the game developed I was targeted by P2 and P3 at the same time in the Mexico zone. I was attacked by two different players from two different fronts in consecutive turns, despite that I didn't complained. Is part of the game attack the weakest link first, isn't it?
After that, I focused on attacking those players and take the south. And guess how was on the south? that's right, P2 and P3 (also P1 but he didn't complained so I decided to avoid it in my explanation).
I didn't attack P4 on some of its few territories on the south since the more territories it controlled the more fight it can do on the north. While they were fighting in the north I strengthened in the south. That was my strategy. Isn't that the game all about?
After P2 and P3 plans didn't went as they expected they started to complain, and actually they explicitly called for an alliance against P4 and me.